In the present study, biochemical and immunochemical methods were used to investigate the presence and distribution of GABA, glutamate decarboxylase (GAD), GABABR1 and GABAA 2 subunit receptors and the vesicular GABA transporter (vGAT) in the cyprid of Balanus amphitrite (=Amphibalanus amphitrite). GAD65/67 immunoreactive neuron cell bodies and nerve fibers were detected in the central nervous system. Paired GAD65/67 immunoreactive nerves running from the posterior ganglion to the body and limb muscles were detected. Thin GABA-immunoreactive nerve terminals were present on striated muscular fibers and in the antennules. Furthermore, GABA, GAD65/67, GABABR1 and GABAA2 subunit receptors and vGAT were observed in the lateral compound eyes, and GABAA2 subunit receptor immunoreactivity was seen in the naupliar eye. These results suggest a neurotransmitter/neuromodulatory role for GABA in thoracic muscle contraction and regulatory functions in compound eyes and antennules of B. amphitrite cyprids.